Search engine optimisation is the work of appearing in unpaid search results when someone looks for what the business does.
For a local trade, most of it is not the website
The results that matter for "electrician near me" are the map listings, and those are driven by the business profile rather than by the site.
The profile itself — complete, correct category, real photographs, service area, hours. See Google Business Profile.
Reviews — volume and recency both count toward ranking, not only the average.
Consistent name, address and phone everywhere the business is listed. Inconsistency between listings is one of the more common reasons a business ranks below where it should.
Proximity to the searcher, which cannot be changed and explains much of what looks inexplicable about local rankings.
What the website contributes
Pages that answer what customers actually search for, one subject per page, in plain language.
A separate page per service and per area served, rather than one page listing everything.
Loading quickly on a phone, since most local searches are made on one.
Backlinks and citations
Links from other sites, and mentions in directories, both signal that the business exists and is established. Quality matters more than quantity, and bought links can be actively harmful.
For a local business the useful ones are ordinary: trade associations, suppliers, local organisations, chambers.
Timescale
Months, not weeks. This is the practical difference from pay-per-click, which starts immediately and stops immediately.
The trade-off is that search results keep working after the spending stops, which makes their effective cost per lead fall over time while advertising's does not.
What to be wary of
Guaranteed rankings. Nobody controls the ranking, so nobody can guarantee it.